Attestation changes + persistent committee changes (#1294)

* Minimal attestation simplification

* minor fix

* Make the tests pass

* Decrease `PLACEHOLDER`, Use `compute_epoch_of_shard_slot`

* Fix proposer signature name and use get_seed() to calculate current_shuffling_seed

* Fix linter error

* Add the WIP `test_is_valid_shard_block`

* Add `get_shard_block_attester_committee`

* Simplified committee selection

* Added some helpers and simplified

* Update specs/core/1_shard-data-chains.md

* Update 1_shard-data-chains.md

* Simplified switchover epochs, changed block structure, changed crosslink structure

* Update 1_shard-data-chains.md

* Moved balance dependency to proposer selection

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Danny Ryan <dannyjryan@gmail.com>

* Update specs/core/1_shard-data-chains.md

* Fixed shard header flattening

* Update specs/core/1_shard-data-chains.md

* Minor fixes

* Update specs/core/1_shard-data-chains.md

* Update specs/core/1_shard-data-chains.md

Co-Authored-By: Hsiao-Wei Wang <hwwang156@gmail.com>

* cleanup testing and lint

* return none if not active validators in persistent committee

* only allow active validators as shard proposer

test_libs/pyspec/eth2spec/test/helpers/phase1/shard_block.py

@@ -0,0 +1,47 @@
+from eth2spec.test.helpers.keys import privkeys
+from eth2spec.utils.bls import (
+    bls_sign,
+    only_with_bls,
+)
+from eth2spec.utils.ssz.ssz_impl import (
+    signing_root,
+)
+
+
+@only_with_bls()
+def sign_shard_block(spec, state, block, shard, proposer_index=None):
+    if proposer_index is None:
+        proposer_index = spec.get_shard_block_proposer_index(state, shard, block.core.slot)
+
+    privkey = privkeys[proposer_index]
+
+    block.signatures.proposer_signature = bls_sign(
+        message_hash=signing_root(block),
+        privkey=privkey,
+        domain=spec.get_domain(
+            state,
+            spec.DOMAIN_SHARD_PROPOSER,
+            spec.compute_epoch_of_shard_slot(block.core.slot),
+        )
+    )
+
+
+def build_empty_shard_block(spec, state, slot, shard, parent_root, signed=False):
+    if slot is None:
+        slot = state.slot
+    block = spec.ShardBlock(
+        core=spec.ExtendedShardBlockCore(
+            slot=slot,
+            beacon_chain_root=state.block_roots[state.slot % spec.SLOTS_PER_HISTORICAL_ROOT],
+            parent_root=parent_root,
+        ),
+        signatures=spec.ShardBlockSignatures(
+            attestation_signature=b'\x12' * 96,
+            proposer_signature=b'\x25' * 96,
+        )
+    )
+
+    if signed:
+        sign_shard_block(spec, state, block, shard)
+
+    return block

test_libs/pyspec/eth2spec/test/phase_1/shard_data_chain/test_shard_block.py

@@ -0,0 +1,26 @@
+from eth2spec.test.helpers.phase1.shard_block import (
+    build_empty_shard_block,
+)
+from eth2spec.test.context import (
+    with_all_phases_except,
+    spec_state_test,
+    always_bls,
+)
+
+
+@with_all_phases_except(['phase0'])
+@always_bls
+@spec_state_test
+def test_is_valid_shard_block(spec, state):
+    block = build_empty_shard_block(
+        spec,
+        state,
+        slot=spec.Slot(spec.PERSISTENT_COMMITTEE_PERIOD * 100),
+        shard=spec.Shard(1),
+        parent_root=spec.Hash(),
+        signed=True,
+    )
+
+    # TODO: test `is_valid_shard_block`
+
+    yield 'blocks', (block,)